The present invention is generally related to mass media drive controllers, and more particularly to a Overshoot Control for Voltage Source/Impedance Based Write Coil Driver.
The current characteristics that are important are shown in FIG. 1. In particular, the current""s rise time, overshoot, undershoot, and settling time are of interest. The desired characteristics for the coil current are a fast rise time and settling time, a controllable amount of overshoot, and very little undershoot. By achieving these current characteristics, a hard disk drive""s storage capacity can be improved.
A conventional current source based write coil driver is shown in FIG. 2. The coil is modeled has LH and RD, and is connected to four transistors via node HWY and HWX. The connection of the four transistors is known as an H-bridge. The H-bridge controls the direction of the coil current IW. A change in the direction of the write coil current creates magnetic flux changes through the coil.
The impedance seen at nodes HWX and HWY is different. If current is flowing through the coil from HWX to HWY, the impedance at HWX is low, and the impedance at HWY is very high. There are two significant problems with this conventional circuit: (1) the common mode impedance between HWX and HWY is not the same; (2) the differential impedance between HWY and HWX cannot be matched to a transmission lines characteristic impedance.
Because of these problems, the coil current will ring and will be distorted by load reflections at HWX, HWY. The coil current distortion could lead to slow risetime, unpredictable current overshoot, longer settling time, and ringing.
The new voltage source/impedance based writer has a similar common mode impedance on nodes HWX, and HWY. Also, the differential impedance between HWX and HWY can match the transmission line impedance that connects the writer to the coil. This architecture achieves the desired write current characteristics.
The present invention achieves technical advantages as an overshoot circuit for the voltage source/impedance based write coil driver accurately controlling the current through a coil that is used to write data to a magnetic medium. The improved characteristics of the coil""s current determines the response of the coil""s magnetic field, which in turn, effects how the magnetic transitions are written to the medium.